Engineering the Slow Photon Effect in Photoactive Nanoporous Anodic Alumina Gradient-Index Filters for Photocatalysis

Abstract

In this study, we explore for the first time the capabilities of nanoporous anodic alumina gradient-index filters (NAA-GIFs) functionalized with titanium dioxide (TiO) photoactive layers to enhance photon-to-electron conversion rates and improve the efficiency of photocatalytic reactions by "slow photon" effect. A set of NAA-GIFs was fabricated by sinusoidal pulse anodization, in which a systematic modification of various anodization parameters (i.e., pore widening time, anodization period, and anodization time) enables the fine-tuning of the photonic stopband (PSB) of these nanoporous photonic crystals (PCs) across the spectral regions. The surface of NAA-GIFs was chemically modified with photoactive layers of TiO to create a composite photoactive material with precisely engineered optical properties. The photocatalytic performance of TiO-functionalized NAA-GIFs was assessed by studying the photodegradation of three model organic dyes (i.e., methyl orange, Rhodamine B, and methylene blue) with well-defined absorption bands across different spectral regions under simulated irradiation conditions. Our study demonstrates that when the edges of characteristic PSB of TiO-modified NAA-GIFs are completely or partially aligned with the absorption band of the organic dyes, the photodegradation rate is enhanced due to "slow photon" effect. A rational design of the photocatalyst material with respect to the organic dye is demonstrated to be optimal to speed up photocatalytic reactions by an efficient management of photons from high-irradiance spectral regions. This provides new opportunities to develop high-performing photocatalytic materials for efficient photocatalysis with broad applicability.

Rational Management of Photons for Enhanced Photocatalysis in Structurally-Colored Nanoporous Anodic Alumina Photonic Crystals

“…the absorbance band of the organics. These observations are in good agreement with our previous study using NAA gradient-index filters as model photonic crystal platforms 4 , which is a further evidence to support our conclusions. Table 3 summarizes the kinetic constants estimated for TiO 2 -NAA-DBRs produced with T P = 900, 1200, and 1600 s calculated from linear fittings, which were 0.15 ± 0.01, 0.14 ± 0.01, and 0.03 ± 0.00 h -1 , respectively.…”

“…visible and IR). 1,4 Nanoporous semiconductor PC structures are superior to their nanoparticle-based counterparts in light-driven applications. Particulate systems have a propensity to aggregate 13 and are also difficult to be recovered after use [14][15] , becoming potentially toxic to the environment 16 .…”

“…4,[33][34][35][36] Significant effort has been devoted to developing alternative nanoporous semiconductor PC structures capable of addressing these limitations to achieve enhanced performances for photocatalysis. Amongst these, nanoporous PCs produced by electrochemical anodization of valve metals have shown promising results in photocatalysis, including titanium dioxide nanotubes [37][38][39][40] and nanoporous anodic alumina (NAA) 4 . Anodization is an industrially scalable, eco-friendly, highly-controllable, and low-cost electrochemical process broadly used in metal finishing industry to modify the properties of metals such as color, brightness, wettability, and corrosion protection.…”

Stacked Nanoporous Anodic Alumina Gradient-Index Filters with Tunable Multispectral Photonic Stopbands as Sensing Platforms

“…28,29 . Furthermore, NAA has high chemical and physical stability, provides stable optical signals without further surface passivation and its surface chemistry can be easily modified with a broad range of functional molecules for multiple applications, including chemo-and biosensing, drug delivery, optical encoding, theranostics implants 30,31 , catalysis and electromagnetism 32,33 .…”

Nanoporous Anodic Alumina Photonic Crystals for Optical Chemo- and Biosensing: Fundamentals, Advances, and Perspectives

“…However, sol-gel chemistry modification is an extensively used method to tune the surface characteristics of NAA platforms and to synthesize nanocomposite materials. The flexibility of sol-gel chemistry to modify NAA platforms is demonstrated by the wide range of materials that can be deposited by this technique such as metal oxides (i.e., titanium dioxides [ 223 ], silica [ 224 ], and zinc oxide [ 224 ]) as well as mixtures of inorganics [ 225 , 226 ]. The surface modification of NAA by sol-gel can be further expanded via the immobilization of molecules onto the sol-gel derived films.…”